This invention relates to a manuscript reading device for use in a picture image inputting portion of a facsimile apparatus, etc., and more particularly to a manuscript reading device of contact type wherein a picture image drawn on a manuscript is read with the manuscript being in contact with the device.
A manuscript reading device of the contact type typically comprises a photodiode array including multiplicity of photodiodes arranged in row, each being biased by a predetermined voltage and storing in parasitic capacitance thereof a signal electric charge which corresponds to lightness-and-darkness information on each minute area of the picture image, and a switching transistor array including a plurality of switching transistors, each connected in series to the corresponding photodiode of the photodiode array, for sequentially outputting the signal electric charge stored in each parasitic capacitance by the switching operation thereof as picture image information.
As is well known, in the contact type manuscript reading apparatus, the length of the photodiode array is set to be the same as the width of the manuscript, wherein the picture image on the manuscript is read in one-to-one image formation manner using such an optical system as an optical fiber array or a lens array, etc. It is also known that by shortening the length of a light transmission passage necessary for forming a picture image, inputting portion of the facsimile apparatus is made considerably compact.
In a conventional manuscript reading device, by using a MOSFET array as a switching transistor array, a higher reading speed can be obtained. However, when mounting ICs consisting of the MOSFET to the substrate in which the photodiode array is provided, the ICs should be connected with the switching transistor array by a wire bonding method or the like. Consequently, the reliability of the device is reduced and a production cost is made high.
To obviate the foregoing problems, the following configuration and driving method has been studied recently;
(a) thin film transistors as a substitute for the MOSFETs, are connected to each photodiode in the photodiode array, the thin film transistor being formed by the same thin film forming process as used in manufacturing the photodiode array;
(b) pairs of photodiode and a thin film transistor, whose one terminal (e.g., drain electrode) is connected to the photodiode, are divided into a plurality of blocks, and the other terminal (e.g., source electrode) of the thin film transistors is connected in a so-called matrix wiring manner in which each block is commonly connected to one wiring group;
(c) the plurality of blocks of the thin film transistors are sequentially driven at a predetermined interval with each block being concurrently driven, whereby signal electric charge of each block temporarily stored in the capacitance of the matrix wiring is sequentially outputted by means of switching transistors constructed by MOSFET.
With this configuration, the number of ICs mounted to the substrate by wire bonding method, etc. can be greatly decreased, and therefore the apparatus can be manufactured quite readily. Furthermore, since the process of wire bonding is reduced, the manufacturing cost is reduced and the reliability of the apparatus is enhanced.
However, the reading speed of the apparatus is lowered because a switching speed of the thin film transistor serving as a transferring transistor is lower than that of the MOSFET.
It is an object of this invention to improve the reading speed and more particularly to a transfer speed of electric charges of the contact type manuscript reading device in which thin film transistors are used as a transistor array for transferring electric charges.